More particularly, the invention relates to an annular expansion ring centered on a main axis and suitable for mounting coaxially on a fuel injector, the ring presenting holes that are distributed around said main axis, that open out into its upstream face, and that enable air to pass towards the zone that is downstream from the ring.
As shown in FIG. 5, which represents the prior art, fuel is injected into a combustion chamber 100 (e.g. into a combustion chamber of a turbomachine) via an injector 10 situated at the end of a pipe delivering the fuel. The injector 10 is substantially cylindrical, and it possesses an annular expansion ring 220 about a main axis A and surrounding a portion of the injector 10, the injector being coaxial with the expansion ring. The expansion ring 220 comprises an axial cylindrical portion 222 with its radially inner surface in contact with or close to the outer surface of the injector 10. The role of the expansion ring 220 is to take up clearance between the injector 10 and the elements constituting the end wall of the combustion chamber, said clearance being caused by the thermal stresses to which these parts are subjected. During combustion, coke can become deposited on the downstream end 12 of the injector 10 as a result of incomplete combustion of the fuel. Deposits of coke are undesirable since they degrade the spraying of fuel by the injectors 10.
Throughout the description, and unless specified otherwise, the adjectives “upstream” and “downstream” are used relative to the normal flow direction of the fuel leaving the injector, i.e. from left to right in FIG. 5. The adjectives “inner” and “outer” relate to proximity relative to the main axis A.
In order to prevent such deposits from occurring, the expansion ring 220 is pierced by holes 226 that are directed substantially axially (i.e. along the direction of the main axis A) and that enable air to penetrate axially into the zone situated downstream from the injector 10. This air thus penetrates parallel to the circumferential side wall of the injector in the upstream zone thereof and forms a layer or film of air around the injector, thus serving to prevent coke from depositing on the downstream end of the injector. In FIG. 5, these holes 226 are formed through the radial wall 224 of the expansion ring 220 that extends the downstream end of the cylindrical portion 222 of the ring radially outwards.
In-service use and tests carried out by the Applicant have nevertheless shown that such a film of air gives rise to drawbacks. The parts constituting the combustion chamber end wall are situated immediately downstream from the injector. This applies in particular to the primary swirler 40 and to the venturi 50. Thus, the primary swirler 40 is an annular part placed coaxially about the injector 10, immediately downstream from the expansion ring 220, and it has an inside diameter that is greater than the diameter of the injector. The primary swirler 40 is pierced all around its circumference by primary holes 42, via which air penetrates into the zone situated downstream from the injector 10. The primary holes 42 are oriented in such a manner that their axes lie in a plane that is radial relative to the main axis, each having an angle of inclination relative to the circumference. Thus, the air leaving the primary holes 42 penetrates into the zone downstream from the injector 10, while turning around the main axis A, thereby forming a swirl or vortex. Immediately downstream from the primary swirler 40, there is the venturi 50 that is an annular part placed coaxially about the injector 10. The venturi 50 possesses a radial wall that is extended downstream (from its inner end) by a converging portion 52, i.e. a conical wall that tapers downstream towards the main axis A. The converging portion 52 is extended by a throat 54 and then by a diverging portion 56 that flares downstream. The converging portion 52 is thus situated downstream from the injector 10, and is situated substantially axially in line with the holes 226 in the expansion ring 220.
The tests carried out by the Applicant have shown that the air from the holes 226 penetrating into the zone downstream from the injector 10 (and from the ring 220) creates turbulence. The present invention seeks to remedy those drawbacks, or at least to attenuate them.